There are various known lighting means for supplying radiant energy from an external source to the interior of a pressure vessel. There are several required criteria for these units. They must provide the necessary light intensity for proper viewing of the vessel contents and yet must be safe to use. Conventional lighting such as bulbs and normal household electrical units obviously cannot be used and therefore light pipes that transmit light from a light source have found acceptance in pressure vessel use. The light pipes should optimumly be explosion-proof, have proper intensity and be substantially resistant to the vessel contents. Many known fiber optic materials that transmit energy and light from a light source have been used as the primary material in these light pipe units. In some lighting systems a bulb is shined through a window in the pressure vessel tank and a great deal of the light is reflected away. So, while these type systems are substantially expolsion-proof and safe, they suffer from the drawback that a substantial portion of the light intensity is lost.
Since many corrosive liquids and gases are used. in pressure vessels and since many pressure vessels are used outdoors, it is important that the light pipe unit be as corrosion resistant as possible. In Applicant's company's U.S Pat. No. 3,813,514 a light pipe unit is disclosed wherein the pipe supplies radiant energy to the interior of a pressure vessel having an internal operating pressure differing from ambient pressure external to the vessel. That disclosed unit had an elongated optical light pipe with an integral frusto-conical shaped mounting surface on one terminal portion and a rounded roughened opposite terminal portion. The pipe extends beyond the internal wall of the pressure vessel and has an inner diffuser end portion to achieve proper light dispersion. After prolonged use this type of light unit becomes corroded because of the deleterious nature of the vessel components and requires maintenance or replacement. A lighting unit compatible and adaptable to existing vessels and light (including U.V and I.R light) with improved corrosion resistance has become required but heretofore not been available.